The use of microwave energy to heat and cook comestibles has been an unqualified commercial success. Today, it is very hard to find an American home without a microwave oven.
As commonplace as the microwave oven has become, however, it is exceptionally surprising to observe the paucity of such heating devices for other household and industrial uses.
For example, as early as 1969, a method and apparatus was suggested for drying and sterilizing fabrics, as illustrated in U.S. Pat. No. 3,605,272, issued: Sept. 20, 1971.
The drying of wet fabrics should have become a commercial reality after fifteen years of research.
One of the drawbacks of perfecting a microwave clothes dryer has been the power requirements. Unlike a microwave oven, which requires a magnetron that generates 400 to 800 watts of microwave power, a typical clothes dryer needs a magnetron generating in excess of two kilowatts. A single magnetron generating this amount of power is very expensive.
Another possible problem with suggested microwave clothes dryer designs, is the inability to transfer and/or distribute the generated power uniformly to the wet fabric. Often hot spots develop in the fabric mass. Such hot spots can cause scorching of the fabric, and are a fire safety concern.
To the best of our knowledge, it never has been suggested that more than one magnetron be utilized to improve heating and drying uniformity. Using two or more magnetrons would solve the first aforementioned problem, wherein several low cost magnetrons could efficiently replace one expensive unit.
However, a clothes dryer with two or more magnetrons would not necessarily be more efficient in the transfer or distribution of the microwave energies. Magnetrons whose generated waves share the same plane of propagation will interfere with each other. Also, unabsorbed power that reflects off the heating chamber walls can enter the wave guide of an adjacent magnetron through its antenna and alter its operation and efficiency.
Another complex problem arises in sensing the dry condition of fabric in a heating chamber having multiple magnetrons. In the past, the dry condition was sensed by the pattern of microwave reflections from a single generator within the drying chamber. Such a sensing system is illustrated in prior U.S. Pat. Nos. 3,290,587; issued Dec. 6, 1966; 3,439,431; issued: April 22, 1969; and 3,192,642; issued: July 6, 1965.
The above sensing techniques presented a fairly uncomplicated approach to the problem of determining the dry condition, mainly because the reflective pattern of a single magnetron or other stand alone microwave generator was easily determinable.
However, with the need for a multiplicity of magnetrons spread in different planes about the drying chamber of the present invention, the pattern of wave reflection is more complicated.
The sensing of the dry condition in the fabric is easily determined by the present invention using but not limited to any or all of the following methods:
1. After continuously measuring relative humidity determining when the chamber outlet relative humidity returns to within approximately 6% above the inlet relative humidity reading.
2. After continuously measuring absolute humidity in the exhaust determining when the absolute humidity output (measured in millivolts) reaches approximately 1 mv above a baseline reading chosen at the beginning of the drying run.
3. Sensing when the chamber exhaust temperature which is continuously measured, shows a sharp increase.
4. Sensing a sharp or sudden increase in continuously measured anode temperature of each magnetron in the chamber.
Each magnetron anode temperature can be used for end point determination of the drying process. However, each magnetron anode temperature sensor can be considered as a potential sensor for a power management scheme in the microwave dryer as well as a method of determining the drying end point. Power management may lead to significant improvements in power efficiency and magnetron life expectancy.